Introduction
There are in general two types of machinery used either to do work on or to have work done by the compression or expansion of gases. These two generic types of machinery are positive displacement and turbine. The positive displacement type includes various mechanically driven or driving pistons or vane type rotors. A volume of gas is carried at relatively low velocity from one volume to a different one, either larger or smaller depending upon the function of compressor or engine. In the other type of machinery, turbines, the gas flow through blades occurs at a velocity of roughly the speed of sound of the gas. It is well known to those designing such machinery that the turbines can be made more efficient than positive displacement machinery. The reason for this difference in efficiency has frequently been obscure. A knowledge of the source of this inefficiency will allow positive displacement machinery to be designed in a fashion such that the inefficiency or loss is reduced by a significant factor to a minimal value. There is, of course, the well-recognized, additional loss of energy in positive displacement machinery due to the friction between whatever is the displacer, piston or vanes, and the walls of the chamber. The turbine in turn avoids this inefficiency but has others such as the friction of aerodynamic flow at velocities near the sound speed.